If the client cannot sit long seated without having to support themselves with their hands on the table that might suggest that their hamstrings are short.
As the pelvis is pulled into a posterior tilt the center of gravity will move backwards in the process. Weak abdominal muscles will be straining to pull the upper torso forward to balance the center of gravity but are often not strong enough to succeed.
Although this does per se not affect actual spinal flexion (lumbar and thoracic spines) it provides interesting information on possible hamstring shortness and / or the strength of the abdominal muscles even before the test starts.
The clients also might get uncomfortable in the quadriceps or hip flexor muscles when sitting long seated, as those have to strain to make up for short hamstrings or erector muscles.
The latter issues should disappear during PROM when the client is pushed forward by the therapist.